1. Field of the Invention
The present invention relates to an image processing apparatus which executes image processing of encoded data that is obtained by compressing an image into units of predetermined blocks, a control method, and a computer-readable medium.
2. Description of the Related Art
In recent years, there has been a high demand for high-resolution color images. In order to meet demands to enhance the image quality of such color images, a digital multifunction peripheral (MFP) frequently handles images having a resolution of 1200 dpi (dots per inch) or higher. In image processing apparatuses (a digital camera and facsimile apparatus) which handle these images in addition to the digital MFP, in order to save memory/hard disk amounts and to shorten write time periods in them, color image data is compressed to attain a cost reduction and high-speed processing.
Conventionally, as a compression method of a color still image, a JPEG method using discrete cosine transformation and a method using Wavelet transformation are popularly used. An encoding method of this type generally encodes an image into units of predetermined blocks (for example, 8×8 or 16×16 pixels), and discrete cosine transformation, quantization, and entropy encoding are applied to the encoded data, thus obtaining a high compression efficiency. Since the encoding method of this type is a variable length encoding method, the code size changes for each image to be encoded.
As the resolution becomes higher, the number of pixels that require image processing increases dramatically, thus increasing its processing load. For example, when the resolution doubles from 600 dpi to 1200 dpi, the number of pixels to be processed increases four times. When the aforementioned image compression is used, in order to refer to and convert that pixel data, decoding processing of compressed data is required. For this reason, processing has to be applied to all pixels of high-resolution data for each pixel, resulting in a long processing time.
As a technique for executing compression processing of pixel data, for example, a known runlength compression method which stores pixel data and its runlength, and a technique for compressing pixel data by detecting an edge for each block, and storing two colors included in that edge (Japanese Patent Laid-Open No. 10-257488) have been disclosed. As a technique for applying image processing to compressed data without any decoding processing, a technique for setting DCT coefficients of, for example, MPEG image data other than a DC component to be zero so as to fill each block by one certain color (Japanese Patent Laid-Open No. 10-290391) has been disclosed.
An electrophotography printer takes a measure against a tailing-blur phenomenon caused by scattered toners on the downstream side of a convey direction of a print sheet due to explosion and scattering of toners at the time of fixing. A conventional printer executes, as a measure against this tailing-blur phenomenon, image processing for detecting a line defined by successive pixels in a main scan direction in an image, and thinning out pixels which belong to a line lower part (downstream side on a print sheet). The high resolution processing of the tailing-blur phenomenon measure is requested as well as other image processing. In order to suppress an increase in memory use amount and a decrease in processing speed while executing resolution enhancement in the tailing-blur measure, image processing is required to be applied to compressed data itself. However, in the related art, image processing cannot be applied to compressed data itself in processing that requires to change an image processing method depending on a target image feature (a line or not) like in the tailing-blur measure.